1. Field of the Invention
The fields to which this invention applies include the agricultural, marine product and food processing industries.
2. Description of the Prior Art
Perishable foodstuffs such as raw, frozen and canned meat, poultry and fish are an important part of the diets of most people and of the international trade of many nations. The degree of freshness of these foods affects their marketability and edibility. Spoiled foods present the risk of illness.
Fish, for example, lose its freshness more quickly than meat. Further, the quality of canned salmon, tuna, crab and the like depends on the freshness of the fish or shellfish used for processing. The quality of freshness can seldom be visually determined because fish is often sold in frozen or processed form.
Perishable foodstuffs imported into the United States, for example, are subject to stringent quality inspection by the Food and Drug Administration. It has been found that up to 30% of canned fish exported from Japn, for example, is spoiled, resulting in the necessity of disposing of the spoiled products.
From the standpoint of consumer protection and food hygiene, detailed studies have been made in Japan of methods for determining the degree of freshness of fish. Based on the fact that the compounds derived from necleic acid-related compounds contained in a fish extract solution undergoes change as shown in the following equation (1), the studies have established a relationship between each compound ratio and the freshness of fish: EQU ATP.fwdarw.ADP.fwdarw.AMP.fwdarw.IMP.fwdarw.HxR.fwdarw.Hx (1)
For example, Uchiyama et al. (Bulletin of the Japanese Society of Scientific Fisheries, Vol. 36, 977 (1970)) made an analysis of the above compounds by using liquid chromatography to show that a deterioration in freshness can be detected from an increase in the value of K as in equation (2) ##EQU1##
Fujii et al. (Ibid., Vol. 39, 69-84 (1973)) determined each amount of IMP, HxR and Hx by using an enzymatic method to estimate the degree of freshness depending on the type of the fish from the numerical values shown in formulas (3) to (5): ##EQU2##
The IMP ratio exhibits a higher value when the degree of freshness is high and decreases as the degree of freshness decreases. Canned tuna having an IMP ratio of 40% or higher can be judged as having been processed from raw tuna having a high degree of freshness.
The compounds ATP, ADP, AMP, IMP and Hx shown in the above equations (1) to (5) represent the following:
ATP: adenosine triphosphate PA1 ADP: adenosine diphosphate PA1 AMP: adenosine monophosphate. PA1 IMP: inosinic acid PA1 HxR: inosine PA1 Hx: hypoxanthine
The above methods for determining the degree of freshness of fish are also effective as measures for determining the degree of freshness of poultry such as chicken (see Numata et al., Journal of Japanese Society of Food Science and Technology, Vol. 28, 542-(1981); and Kitada et al., Ibid., Vol. 30, No. 3, 151-154 (1983)).
Both of the above methods, however, need to be carried out in specially equipped laboratories by skilled personnel.